+ 'Polyester microfibers are one of the most abundant microplastics in marine ecosystems, yet their observed near-surface concentrations are inconsistent with expectations based on their density. To study the mechanisms governing polyester microfiber transport and fate, we developed a Lagrangian particle-tracking module using Parcels to simulate microfiber dynamics in the Salish Sea, incorporating realistic hydrodynamics from a three-dimensional circulation model, SalishSeaCast. The model simulates microfiber release from wastewater treatment plant outfalls across the Salish Sea. A multi-month simulation ensemble highlights strong estuarine retention and shows sedimentation is the primary sink for our configuration. On average, 31 13 of released microfibers are sedimented, 14 4 are beached, but only 0.13 0.11 are exported to the Pacific Ocean. Our model concentration estimations agree within an order of magnitude with new near-surface observations near the Fraser River, but regional discrepancies indicate the existence of additional sources. The results indicate that polyester microfibers accumulate close to source locations and behind sills, due to estuarine dynamics. This model can be used to assess microfiber fate in coastal environments and can inform future pollution mitigation strategies.',
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